1. Field of the Invention
The present invention relates to a synthetic fused silica member used, for example, in an exposure apparatus (optical aligner) for producing IC in which a light source providing light having a wavelength of 200 nm or less such as fluorine excimer laser or the like is used, especially a synthetic fused silica substrate for photomask.
2. Description of the Related Art
Along with a recent increase in the degree of integration of semiconductor device, there has been required for improving accuracy of micromachining in so-called photolithography technique. In order to make fine patterns, it is necessary to use a light source providing a light having shorter wavelength, and higher energy. Therefore, high transmittance and durability to light having short wavelength have been required in optical member to be used.
Conventionally, there have been used synthetic fused silica or fluorite (CaF.sub.2) as a photomask, lens or the like in an exposure apparatus for producing semiconductor IC or the like in which a light source providing light having short wavelength, as in ultraviolet region or vacuum ultraviolet region, such as excimer laser or the like is used. Because, only synthetic fused silica and fluoride crystal as represented by fluorite can be practically used as the material having high transmittance to light having wavelength such as KrF (248 nm), ArF (193 nm) excimer laser.
The characteristics of the synthetic fused silica are different depending on methods for producing it. For example, there are the synthetic fused silica having high hydroxyl group-content which is produced from oxyhydrogen flame by "a direct method" and those having low hydroxyl group-content produced by "a soot method" wherein silica sintered body is molten.
As described above, along with increase in a degree of integration of semiconductor devices, a light source providing a light having a shorter wavelength, represented by KrF (248 nm) or ArF (193 nm) excimer laser has been used, and as a result, a photon energy of the light has been increased. When a synthetic fused silica is irradiated with a light having a short wavelength and high photon energy, structural defects which can be substantial damages may be caused in the glass, and the transmittance may be lowered. There is proposed the material in which significant lowering of transmittance is not caused, even when it is irradiated with a light having short wavelength and high photon energy (see, for example, in Japanese Patent KOKAI Hei 7-291635).
However, recently, optical member for light in the vacuum ultraviolet region which has shorter wavelength as 200 nm or less, especially for fluorine excimer laser (157 nm), have been developed. If the optical member used for such a light source having short wavelength can be made of a synthetic fused silica, it would be quite advantageous, since the technology is an extension of the conventional optical exposure technology, and only slight change of processes, for example, change of light source is necessary. Namely, the conventional photolithography technology can be prolonged.
However, the above-mentioned range of the wavelength is close to essential absorption edge of transmittance of the synthetic fused silica. Accordingly, if it is pure SiO.sub.2, it is in principle transparent in the range of the wavelength up to 125 nm (see Material Technology-Highly Functional Glass, published by Tokyo University publishing department). However, as described above, an actual synthetic fused silica contains hydroxyl groups, chlorine, metal impurity, or the like depending on the method for production, and absorption due to them has a subtle affect on the transmittance.
On the other hand, fluorite may be used as other material than synthetic fused silica that is excellent in transmittance in the vacuum ultraviolet region. However, it has a coefficient of thermal expansion that is double-digits higher than that of synthetic fused silica. For example, when it is used as a substrate for a photomask, accuracy of dimension gets worse due to affect of thermal expansion, which may lead to blur of patterns. Furthermore, it is disadvantageous that water cannot be used in the steps for polishing a member made of fluorite to be transparent, since fluorite is deliquescent. Accordingly, it is difficult to be machined.